Method of forming predominantly &lt;100&gt; polycrystalline silicon thin film transistors

ABSTRACT

A method is provided to produce thin film transistors (TFTs) on polycrystalline films having a single predominant crystal orientation. A layer of amorphous silicon is deposited over a substrate to a thickness suitable for producing a desired crystal orientation. Lateral-seeded excimer laser annealing (LS-ELA) is used to crystallize the amorphous silicon to form a film with a preferred crystal orientation. A gate is formed overlying the polycrystalline film. The polycrystalline film is doped to produce source and drain regions.

CROSS-REFERENCE

This application is a continuation of application Ser. No. 09/796,341,filed Feb. 28, 2001, entitled “Method of Forming Predominantly <100>Polycrystalline Silicon Thin Film Transistors,” invented by Apostolos T.Voutsas.

BACKGROUND OF THE INVENTION

This invention relates generally to semiconductor technology and moreparticularly to the method of forming thin film transistors (TFTs) onpolycrystalline silicon regions within an amorphous silicon film.

Polycrystalline silicon is formed by crystallizing amorphous siliconfilms. One method of crystallizing amorphous silicon films is excimerlaser annealing (ELA). Conventional ELA processes form polycrystallinefilms having a random polycrystalline structure. Random, as used here,means that no single crystal orientation is dominant and thatpolycrystalline structures consist of a mixture of crystallographicorientations in silicon. These crystallographic orientations in siliconare commonly denoted as <111>, <110>, and <100>, along with theirrespective corollaries, as is well known in the art. Control ofcrystallographic orientation is generally desirable because theelectrical characteristics of a polycrystalline silicon film depend uponthe crystallographic orientation of the film. In addition, theuniformity of the electrical characteristics will improve if themajority of the film has a controllable texture.

ELA, as well as many other annealing methods, has not provided a meansto control these microstructural characteristics and achieve apredictable and repeatable preferential crystal orientation and filmtexture within an annealed film. It would be desirable to have a methodof producing TFTs using a polycrystalline silicon film with a moreuniform crystallographic orientation. It would also be desirable to beable to produce TFTs using predominantly <100> polycrystalline silicon.

SUMMARY OF THE INVENTION

Accordingly, a method of forming thin film transistor (TFT) structureson a substrate, which has a polycrystalline silicon film with a desiredpredominant crystal orientation, is provided. The method of forming theTFTs comprises the steps of: providing a substrate, depositing anamorphous silicon film on the substrate, annealing the substrate toproduce a polycrystalline film with the desired predominant crystalorientation, preferably a <100> crystal orientation, forming a gatestructure over the polycrystalline film; and doping the polycrystallinefilm to produce source regions and drain regions.

The substrate can be any material that is compatible with the depositionof amorphous silicon and excimer laser annealing. For displayapplications, the substrate is preferably a transparent substrate suchas quartz, glass or plastic.

To achieve a good quality film that is predominantly <100> crystalorientation, the step of depositing the amorphous film should deposit toa thickness of at least approximately 100 nm.

The step of annealing preferably uses a laterally seeded excimer laserannealing process.

The method of the present invention, produces a thin film transistorstructure comprising a polycrystalline film, which has a predominantly<100> crystal orientation, overlying a substrate. The final film is atleast 100 nm thick. A gate structure overlies the polycrystallinesilicon film and source/drain regions are formed by doping thepolycrystalline silicon film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an excimer laseranneal (ELA) apparatus used in connection with the present method.

FIG. 2 (prior art) is a cross-sectional view showing polycrystallinefilm crystallized using an interface-seeded ELA (IS-ELA) process

FIG. 3 illustrates a step in the process of lateral-seeded ELA (LS-ELA).

FIG. 4 illustrates a step in the process of lateral-seeded ELA (LS-ELA).

FIG. 5 illustrates a step in the process of lateral-seeded ELA (LS-ELA).

FIG. 6 is a scatter plot of crystal orientations for a 35 nm thick film.

FIG. 7 is a scatter plot of crystal orientations for a 45 nm thick film.

FIG. 8 is a scatter plot of crystal orientations for a 75 nm thick film.

FIG. 9 is a scatter plot of crystal orientations for a 100 nm thickfilm.

FIG. 10 is a diagram illustrating variation in crystal orientation forvarious film thicknesses.

FIG. 11 is a flowchart of a process of performing the method of thepresent invention.

FIG. 12 is a cross-sectional view of a substrate during processing.

FIG. 13 is a cross-sectional view of a substrate during processing.

FIG. 14 is a cross-sectional view of transistor structures formed on thesubstrate during processing.

FIG. 15 is a cross-sectional view of transistor structures formed on thesubstrate during processing.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 a lateral-seeded excimer laser annealing (LS-ELA)apparatus 10 is shown. LS-ELA apparatus 10 has a laser source 12. Lasersource 12 includes a laser (not shown) along with optics, includingmirrors and lens, which shape a laser beam 14 (shown by dotted lines)and direct it toward a substrate 16, which is supported by a stage 17.The laser beam 14 passes through a mask 18 supported by a mask holder20. The laser beam 14 preferably has an output energy in the range of0.8 to 1 Joule when the mask 18 is 50 mm×50 mm. Currently availablecommercial lasers such as Lambda Steel 1000 can achieve this output. Asthe power of available lasers increases, the energy of the laser beam 14will be able to be higher, and the mask size will be able to increase aswell. After passing through the mask 18, the laser beam 14 passesthrough demagnification optics 22 (shown schematically). Thedemagnification optics 22 reduce the size of the laser beam reducing thesize of any image produced after passing through the mask 18, andsimultaneously increasing the intensity of the optical energy strikingthe substrate 16 at a desired location 24. The demagnification istypically on the order of between 3× and 7× reduction, preferably a 5×reduction, in image size. For a 5× reduction the image of the mask 18striking the surface at the location 24 has 25 times less total areathan the mask, correspondingly increasing the energy density of thelaser beam 14 at the location 24.

The stage 17 is preferably a precision x-y stage that can accuratelyposition the substrate 16 under the beam 14. The stage 17 is preferablycapable of motion along the z-axis, enabling it to move up and down toassist in focusing or defocusing the image of the mask 18 produced bythe laser beam 14 at the location 24. The mask holder 20 is also capableof x-y movement.

FIG. 2 illustrates aspects of a prior art ELA process. This process issometimes referred to as Interface-Seeded ELA (IS-ELA). An amorphoussilicon film 30 has been deposited over the substrate 16. A laser pulseis directed at the amorphous silicon film 30, which melts andcrystallizes a region 32. The laser pulse melts a region on the order of0.5 mm. Small microcrystalline seeds 34 remain, or form, at theinterface. As the surrounding amorphous silicon crystallizes these seedsaffect the crystal orientation. Since the seeds 34 have a variety ofcrystal orientations, the resulting films will accordingly have a widemix of crystal orientations. This is illustrated by previouslycrystallized region 36. In actuality, since a large number of seedswould be present at the interface, a large number of crystalorientations would form.

FIGS. 3 through 5 illustrate the steps of Lateral-Seeded ELA (LS-ELA),which is also referred to as Lateral-Growth ELA (LG-ELA) or LateralCrystallization ELA (LC-ELA). Starting with FIG. 3, the amorphoussilicon film 30 has been deposited over the substrate 16. A laser beampulse has been passed through openings 40 in the mask 18 to formbeamlets, which irradiate regions 42 of the amorphous silicon film 30.Each beamlet is on the order of 5 microns wide. This is approximately100 times narrower than the 0.5 mm used in the prior art IS-ELA process.The small regions 42 are melted and crystallized by the beamletsproduced by the laser pulse passing through the mask.

After each pulse the mask 18 is advanced by an amount not greater thanhalf the lateral crystal growth distance. A subsequent pulse is thendirected at the new area. By advancing the image of the openings 40 asmall distance, the crystals produced by preceding steps act as seedcrystals for subsequent crystallization of adjacent material. Referringnow to FIG. 4, the irradiated regions 42 have moved slightly. Thepreviously crystallized regions 44 act as the seed crystal for thecrystallization of the irradiated regions 42. By repeating the processof advancing the mask laterally and firing short pulses the crystal iseffectively pulled in the direction of the advancing laser pulses.

FIG. 5 shows the amorphous silicon film 30 after several additionalpulses following FIG. 4. The crystals have continued to grow in thedirection of the masks' movement to form a polycrystalline region. Themask will preferably advance until each opening 40 reaches the edge of apolycrystalline region formed by the opening immediately preceding it.To crystallize larger regions, the stage 17, which was described inreference to FIG. 1, can be moved, and the mask 18 repositioned, tocontinue crystallizing the amorphous silicon film 30 until a region ofthe desired size has been crystallized.

This LS-ELA process produces crystallized regions that are more uniform,due to the propagation of a first crystallized region by subsequentlaser pulses, as opposed to crystallized regions formed using multipleseed crystals at the interface. FIGS. 6 through 9 are plots thatillustrate the affect of amorphous silicon film thickness on theresulting predominant crystal orientation.

FIG. 6 is a plot of the distribution of crystal orientation for a 30 nmthick deposited amorphous silicon film after LS-ELA processing. FIG. 6shows that a majority of the crystals are in a 101 region 50. The 101region 50 corresponds to a <110> crystal orientation.

FIG. 7 is a plot of the distribution of crystal orientation for a 45 nmthick deposited amorphous silicon film after LS-ELA processing. FIG. 7shows that the crystal orientations are spread throughout theorientation plot. This is a less ideal condition for the resulting film.It should be noted that the predominant crystal orientation has shiftedaway from the <110> orientation toward the <100> orientation region 52,which corresponds to 001 on the plot.

FIG. 8 is a plot of the distribution of crystal orientation for a 75 nmthick deposited amorphous silicon film after LS-ELA processing. FIG. 8shows that the crystal orientation has moved closer to the <100>orientation. However, the crystal orientation is still spread over arelatively wide range of crystal orientations.

FIG. 9 is a plot of the distribution of crystal orientation for a 100 nmthick deposited amorphous silicon film after LS-ELA processing. FIG. 9shows that the crystal orientation is now predominantly <100> as shownby the <100> region 52.

FIG. 10 is a diagram illustrating variation in crystal orientation forvarious film thicknesses. A first thickness 60, which corresponds to anapproximately 35 nm thick film, has a <110> orientation to within lessthan 10 degrees. A second film thickness 62, which corresponds to anapproximately 45 nm thick film, has a mix of <100> orientation to within25 degrees and <101>orientation to within approximately 20 degrees. Athird film thickness 64, which corresponds to an approximately 75 nmthick film, has a mix of <100> orientation to within approximately 20degrees and <101> orientation to within approximately 15 degrees. Aforth film thickness 66, which corresponds to an approximately 100 nmthick film, has a <100> orientation to within approximately 10 degrees.As used herein, the term predominant crystal orientation, or any similarphrase, refers to a material that is within less than 15 degrees of adesired crystal orientation. Looking at FIG. 10, it is apparent that itis possible to produce films with predominantly <110> orientation, or<100> orientation. <100> orientation is generally preferred forsemiconductor processes because of its electrical properties.Unfortunately, to produce predominantly <100> orientation requires theformation of thicker films than those that generally are considereddesirable for the formation of thin film transistors. Thicker films tendto have greater leakage currents than thinner films. In the method ofthis invention, the compromise is made between leakage current and thedesirable electrical properties associated with having predominantly<100> polycrystalline films. While it may be possible to polish thefilms to produce thinner films, this may not be practical for allapplications.

Referring now to FIG. 11, a flow chart of the steps of the method of thepresent invention is shown. Step 110 deposits a layer of amorphoussilicon over the substrate. The layer of amorphous silicon should bethick enough to produce predominantly <100> polycrystalline siliconfollowing subsequent processing according to the method of the presentinvention. The necessary thickness to produce a predominantly <100>polycrystalline material can be determined without undueexperimentation. Preferably, the layer of amorphous silicon will be atleast approximately 100 nm thick.

Step 120 performs lateral crystallization using LS-ELA to produce apolycrystalline region having a predominantly <100> crystal orientation.A laser beam is used to project an image of the mask onto the substrate.The laser beam energy is sufficient to cause amorphous silicon tocrystallize. A sequence of laser pulses can be used to crystallize aregion, as described above. The resulting polycrystalline film ispredominantly <100> crystal orientation, meaning within 15 degrees of<100> crystal orientation. Preferably, the crystal orientation is within10 degrees of <100> crystal orientation.

Step 130 forms a gate stack overlying the polycrystalline film. The gatestack includes a dielectric layer, preferably silicon dioxide, and agate, preferably composed of polysilicon.

Step 140 dopes regions adjacent the gate stack to form n-type and p-typeregions on either side of the gate stack. These doped regions arereferred to as source and drain regions. The doping is accomplished byappropriately masking the area, implanting the desired dopants, andannealing. In TFT structures, the dopants preferably extend through thethickness of the polycrystalline film.

FIGS. 12 through 15 show the film at various stages of processing. FIG.12 shows the substrate 16 with an overlying amorphous silicon film 30.For display applications, the substrate is preferably transparent.Available transparent substrate materials include quartz, glass, andplastic. Although it is not shown, a barrier coat may be used betweenthe substrate and the amorphous silicon as is well known to one ofordinary skill in the art. The amorphous silicon film is preferablythick enough to form a predominantly <100> crystal orientation followingLS-ELA processing. Amorphous silicon films on the order of at leastapproximately 100 nm will produce predominantly <100> crystalorientation. Slightly thinner films may also produce the desired result,without undue experimentation.

FIG. 13 shows a polycrystalline region 44 following the LS-ELA process,which was discussed above. The polycrystalline region 44 ispredominantly <100>. By predominantly <100>, it is meant that theorientation is within 15 degrees of <100> as described above withreference to FIG. 10.

FIG. 14 shows TFT structures 70 formed using the polycrystalline film. Agate 72 has been formed overlying the polycrystalline film, with adielectric layer 74 interposed between the gate and the polycrystallinefilm. Source region 76 and drain region 78 have been formed within thepolycrystalline film by doping the polycrystalline film with n-type andp-type dopants, respectively.

The gate is preferably a polysilicon gate. The interposed dielectriclayer is silicon dioxide, or other suitable dielectric material. Thesource and drain regions are formed by implanting, or other suitabledoping method.

The polycrystalline film is removed from the substrate over areas thatare not used to produce TFTs or other device elements. Elimination ofthe polycrystalline film from these open spaces provides isolation ofdevice components.

Referring now to FIG. 15, an isolation material 80 is provided toisolate the device components as well as the metal connections 82.

Although a simple TFT structure has been shown, many differenttransistor structures are known to those of ordinary skill in the artand could be used in connection with the present method. Lightly-dopeddrain and source regions could be used. A variety of gate structurescould be used as well, including substitute gates, and new high-kdielectric materials. The invention is not limited to the specificembodiments described above, but is defined by the claims.

What is claimed is:
 1. A method of forming thin film transistor (TFT)structures on a substrate comprising the steps of; a) providing thesubstrate; b) depositing an amorphous silicon film at least 100 nm thickover the substrate; c) annealing the amorphous silicon film using alateral crystallization process to produce a polycrystalline film havinga predominantly <100> crystallographic orientation; d) forming a gatestructures over the polycrystalline film; and e) doping thepolycrystalline film having a predominantly <100> crystallographicorientation to produce source regions and drain regions.
 2. The methodof claim 1, wherein the substrate is transparent.
 3. The method of claim2, wherein the substrate is quartz, glass or plastic.
 4. The method ofclaim 1, wherein the amorphous silicon film is in the range of betweenapproximately 100 and 250 nm thick.
 5. The method of claim 1, whereinthe polycrystalline film has a crystallographic orientation within 15degrees of <100>.
 6. The method of claim 1, wherein the polycrystallinefilm has a crystallographic orientation within 10 degrees of <100>. 7.The method of claim 1, wherein the lateral crystallization processcomprises a sequence of laser pulses projected through a mask having anarrow slit to project a beamlet onto the surface of the amorphoussilicon film to crystallize the amorphous silicon film as the beamlet isadvanced over the surface of the amorphous silicon film betweensuccessive laser pulses.